Allow "PhutilAWSException" to identify "EBS: Not Found" errors

Summary: Ref T13630. Piledriver cares about EBS error specifics; expand this class a bit to expose more error information.

Test Plan: Created and destroyed resource piles with Piledriver control code, elsewhere.

Maniphest Tasks: T13630

Differential Revision: https://secure.phabricator.com/D21732